Model-based control for torque biasing system

ABSTRACT

A method of controlling a torque biasing system includes determining a torque command, calculating a torque error based on the torque command and a model-based torque. A control signal is generated based on the torque error and the torque biasing system is operated based on the control signal.

FIELD OF THE INVENTION

The present invention relates to torque biasing systems, and moreparticularly to model-based control of a torque biasing system.

BACKGROUND OF THE INVENTION

Torque biasing systems can be implemented in vehicle componentsincluding, but not limited to, a transfer case, a power take-off unit(PTU) and an axle. Torque biasing systems regulate torque transferbetween an input and an output. More specifically, a clutch pack isoperably disposed between the input and the output. The degree ofengagement of the clutch pack is varied to regulate the amount of torquetransferred from the input to the output. For example, when the clutchpack is disengaged, there is no torque transfer from the input to theoutput. When the clutch pack is fully engaged or locked, all of thetorque is transferred from the input to the output. When partiallyengaged, a corresponding portion of the torque is transferred from theinput to the output.

The degree of clutch pack engagement is adjusted by a linear force thatis imparted on the clutch pack via an actuator system. Traditionalactuator systems include an electric motor and a clutch operatormechanism. The clutch operator mechanism converts the torque generatedby the electric motor into the linear force, which can be amplifiedprior to being imparted on the clutch pack. The electric motor iscontrolled based on a control signal generated by a control system.

Conventional control systems use closed-loop control to regulate aspecified system parameter. When the specified system parameter has anaccurate means of feedback, such as is the case with direct sensing, theoverall system accuracy is sufficient. In the case where the specifiedsystem parameter is not directly measurable, system accuracy isdifficult to achieve.

Torque biasing systems are typically controlled based on a parameterother than torque, because torque is not easily measurable and torquesensors are not readily available. Torque sensors, however, would not bea total solution because the actual torque generated by a vehicle systemis often much slower than is required by the biasing device. As aresult, conventional torque biasing systems are not controlled asaccurately as is desired.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a method of controlling atorque biasing system. The method includes determining a torque command,calculating a torque error based on the torque command and a model-basedtorque. A control signal is generated based on the torque error and thetorque biasing system is operated based on the control signal.

In one feature, the method further includes processing a previouscontrol signal through a torque biasing system model to generate themodel-based torque. The torque biasing system model includes a motormodel, a clutch operator model and a clutch model. The control signal isprocessed through the motor model to generate a clutch operatorinterconnection value. The clutch operator interconnection value isgenerated based on a resistance torque, a motor position signal andmotor data.

In still another feature, the method further includes calculating theresistance torque using the clutch operator model. An interconnectionposition value is processed through the clutch operator model togenerate a clutch interconnection value. The clutch interconnectionvalue is generated based on a resistance force and clutch operator data.The resistance force is calculated using the clutch model.

In yet another feature, the method further includes processing a clutchinterconnection value through the clutch model to generate themodel-based torque.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of a vehicle including a transfercase that incorporates an exemplary torque biasing system;

FIG. 2 is a logic diagram illustrating a model-based control systemaccording to the present invention;

FIG. 3 is a logic diagram illustrating a torque biasing system modelaccording to the present invention;

FIG. 4 is a logic diagram illustrating a motor module according to thepresent invention;

FIG. 5 is a logic diagram illustrating a clutch operator moduleaccording to the present invention; and

FIG. 6 is a logic diagram illustrating a clutch module according to thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiments is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses. As used herein, the term module refers to anapplication specific integrated circuit (ASIC), an electronic circuit, aprocessor (shared, dedicated, or group) and memory that execute one ormore software or firmware programs, a combinational logic circuit, orother suitable components that provide the described functionality.

Referring now to FIG. 1, a four-wheel drive vehicle 10 is illustrated.The vehicle includes a front drive line 22, a rear drive line 24, and apower source, such as an engine 26 (partially shown), which providesdrive torque to the front and rear drive lines through a transmission28. The transmission 28 may be either a manual or automatic shiftingtype. The front drive line 22 includes a pair of front wheels 30connected to opposite ends of a front axle assembly 32 having a frontdifferential 34. The front differential 34 is coupled to one end of afront prop shaft 36, the opposite end of which is coupled to a frontoutput shaft 38 of a transfer case 40. Similarly, the rear drive line 24includes a pair of rear wheels 42 connected to opposite ends of a rearaxle assembly 44 having a rear differential 46. The rear differential 46is coupled to one end of a rear prop shaft 48, the opposite end of whichis coupled to a rear output shaft 50 of the transfer case 40. Thetransfer case 40 is equipped with an electronically-controlled torquebiasing system 52 that is operable to control the magnitude of speeddifferentiation and torque distribution between the output shafts 38 and50.

Adaptive actuation of the torque biasing system 52 is controlled by acontrol system that includes a group of sensors 56 for monitoringspecific dynamic and operational characteristics of the vehicle 10 andgenerating sensor signals indicative thereof, and a controller 58 forgenerating control signals in response to the sensor input signals.Moreover, the controller 58 is adapted to control the actuated conditionof torque biasing system 52 by generating digital control signals basedon both the sensor input signals and torque biasing system model of thepresent invention.

A mode select mechanism 60 enables a vehicle operator to select one ofthe available drive modes. In particular, the controller 58 controls thetorque biasing system 52 in response to a mode signal sent to thecontroller 58 from mode select mechanism 60. The mode signal indicatesthe particular drive mode selected. When an “adaptive” four-wheel drivemode is selected, the controller 58 operates to continuously monitor andautomatically regulate the actuated condition of torque biasing system52 between its non-actuated and fully actuated limits, thereby varyingthe magnitude of speed differentiation and torque distribution betweenoutput shafts 38 and 50. When the mode signal indicates that a “locked”four-wheel drive mode has been selected, the torque biasing system 52 isfully actuated, whereby non-differentiated power is delivered to outputshafts 38 and 50. The locked four-wheel drive mode is provided to permitimproved traction when the vehicle is operated off road or over severeroad conditions.

Referring now to FIG. 2, a schematic illustration of the torque biasingsystem 52 is shown. The torque biasing system 52 includes a motor 70, aclutch operator mechanism 72 and a clutch-pack 74. It is anticipatedthat the clutch operator mechanism includes a driven torque/forceconversion device with an amplifier mechanism. Anticipated driversinclude motors or solenoids. Anticipated torque/force conversion devicesinclude cam/follower devices, dual cam plate devices and scissor platesand anticipated amplifier mechanisms include levers and ball ramps. Aninput torque (T_(INPUT)) is transferred through the clutch-pack 74 toprovide an output torque (T_(OUTPUT)). The motor 70 is operated based ona control signal to manipulate the clutch operator mechanism 72. Thegear reduction/shift lever system 72 imparts a linear force on theclutch-pack 74 that regulates engagement of the clutch-pack 74.T_(OUTPUT) is based on the degree of clutch-engagement. The controller58 generates the control signal as discussed in detail below.

Referring now to FIG. 2, the model-based control of the presentinvention will be described in detail. A torque command (T_(COM)) isgenerated based on vehicle inputs. T_(COM) is the amount of torque thatis to be transferred through the torque biasing system 52 and is arunning calculation based on wheel speeds, yaw rate, throttle and thelike. The wheel speeds, yaw rate and throttle signals are generated bythe sensor group 56. A summer 78 generates a torque error (T_(ERROR)) asthe difference between T_(COM) and a model-based torque (T_(CALC)). Themodel-based control is implemented via a motor module 80, a clutchoperator module 82 and a clutch module 84 as described in further detailbelow. More particularly, the motor module 80 is based on a motor model,the clutch operator module 82 is based on a shift system model and theclutch module 84 is based on a clutch model.

A motor control module 86 generates a motor voltage (V_(MOTOR)) based onT_(ERROR) and a motor position signal (M_(POS)). The motor controlmodule 86 is preferably a proportional, integral, derivative (PID)control module of a type known in the art. The motor 70 operates basedon V_(MOTOR) and includes a position sensor 88 and a temperature sensor90. The position sensor 88 generates M_(POS), which indicates therotational position of the motor armature (not shown). The temperaturesensor 90 generates a motor temperature signal (M_(TEMP)). The motor 70generates a torque (T_(MOTOR)) that drives the shift system 72.

The shift system 72 generates a linear force (F) that is imparted on theclutch pack 74. F controls the engagement of the clutch pack 74. Moreparticularly, as F increases, clutch slip is decreased until lock-up isachieved. During clutch slip, the input torque (T_(INPUT)) is greaterthan the output torque (T_(OUTPUT)). At clutch lock-up, T_(INPUT) isequal to T_(OUTPUT). In other words, all of T_(INPUT) is transferredthrough the clutch-pack 74 during clutch lock-up. The clutch-pack 74includes a temperature sensor 92 that generates a temperature signal(C_(TEMP)).

Referring now to FIG. 3, T_(CALC) is determined based on motor data,V_(MOTOR), M_(POS), M_(TEMP), I_(MOTOR), shift system data and clutchdata. More particularly, the motor module 80 determines a physicalcharacteristic of the motor 70 (i.e., armature position) based onelectrical motor characteristics (i.e. the motor data, V_(MOTOR) andI_(MOTOR)) and physical motor characteristics (i.e., M_(POS) andM_(TEMP)). The motor module 80 also accounts for the gear ratios of thegear reduction system. The motor module 80 generates a clutch operatorinterconnection position (P_(COINT)) based on the motor data, V_(MOTOR),M_(POS) and M_(TEMP) and a resistance torque (T_(RES)). T_(RES) isdetermined as discussed in further detail below. P_(COINT) indicates therotational position of the physical component (e.g., screw) thatinterconnects the motor 70 and the clutch operator mechanism 72.

The clutch operator module 82 determines a clutch interconnectionposition (P_(CINT)) based on the clutch operator data, P_(COINT) and aresistance force (F_(RES)). F_(RES) is determined by the clutch model 84as discussed in further detail below. The shift system module 82 alsocalculates T_(RES), which is fed back to the motor module 80. The clutchmodule 84 calculates T_(CALC) based on clutch data, C_(TEMP), wheelvelocities, a nominal kiss point (NOM_(KP)) a corrected kiss point(CORR_(KP)) and P_(CINT). The clutch module 84 also calculates F_(RES),which is fed back to the shift system module 82.

Referring now to FIG. 4, the motor module 80 will be discussed infurther detail. The motor data is provided by the motor manufacturer andincludes a current to torque conversion factor (k_(T)) a back EMFconstant (k_(E)), brake on drag, brake off drag, viscous drag, coilresistance (R_(COIL)), inertia and gear ratio. The motor module 80includes a current calculating module 100, a drag torque calculatingmodule 102, a velocity calculating module 104 and a position calculatingmodule 106. The current calculating module 100 calculates a current (I)based on V_(MOTOR), R_(COIL), k_(E), I_(MOTOR) and an angular velocity(ω_(MOTOR)). ω_(MOTOR) is calculated by the velocity calculating module104 as discussed in further detail below. A multiplier 108 multiplies Iby k_(T) to provide an indicated motor torque (T_(MOTORIND)).

The drag torque calculating module 102 calculates a brake drag torque(T_(DRAGBRK)) and a viscous damper drag torque (T_(DRAGVD)) based onω_(MOTOR), a brake enable signal and the viscous drag motor data. Moreparticularly, T_(DRAGBRK) is calculated based on ω_(MOTOR) and eitherthe brake on drag or the brake off drag motor data. If the brake enablesignal indicates brake on, T_(DRAGBRK) is determined based on the brakeon drag motor data. If the brake enable signal indicates brake off,T_(DRAGBRK) is determined based on the brake off motor data. T_(DRAGVD)is determined based on ω_(MOTOR) and the viscous drag motor data.T_(DRAGBRK) and T_(DRAGVD) are subtracted from T_(MOTOR) by a summer 110to provide an adjusted motor torque (T_(MOTORADJ)).

T_(RES) is subtracted from T_(MOTORADJ) by a summer 112 to provide anacceleration motor torque (T_(MOTORACC)). T_(MOTORACC) is multiplied bythe inertia motor data to provide an angular acceleration (α_(MOTOR)).The velocity calculating module 104 calculates ω_(MOTOR) based onα_(MOTOR) and a time step (t_(K)). The position calculating module 106calculates P_(COINT) based on ω_(MOTOR), M_(POS), t_(K) and the gearratio motor data.

Referring now to FIG. 5, the clutch operator module 82 will be explainedin detail. The clutch operator data includes a spring rate (k_(SPRING)),an efficiency (CO_(EFF)), a drag factor (CO_(DRAG)), a viscous damperdrag factor (CO_(DRAGVD)), a position ratio (CO_(RATIO)) and an inertia(CO_(INERTIA)). The clutch operator module 82 includes a dragcalculating module 114, a velocity calculating module 116 and a positioncalculating module 118. A clutch operator position (P_(CO)) issubtracted from P_(COINT) by a summer 120 to provide a position error(P_(ERROR)). P_(CO) is calculated by the position calculating module 118as discussed below. A multiplier 122 multiplies P_(ERROR) and k_(SPRING)to provide T_(RES).

The drag calculating module 114 calculates a clutch operator torque(T_(CO)) based on CO_(EFF), CO_(DRAG), CO_(DRAGVD), T_(RES) and a clutchoperator angular velocity (ω_(CO)). More particularly, the dragcalculating module 114 updates T_(RES) to account for efficiency lossesand calculates a drag torque and a viscous damper drag torque. The dragtorque and viscous damper drag torque are subtracted from the updatedT_(RES) to provide T_(CO). An inertia torque (T_(INERTIA)) is determinedas the product of F_(RES) and CO_(RATIO) by a multiplier 124.T_(INERTIA) is subtracted from T_(CO) by a summer 126 to provide aclutch operator acceleration torque (T_(COACC)). A clutch operatorangular acceleration (α_(CO)) is determined as the product of T_(COACC)and CO_(INERTIA) by a multiplier 128. The velocity calculating module116 calculates ω_(CO) based on α_(CO) and t_(K). The positioncalculating module 118 calculates P_(CO) based on ω_(CO) and t_(K).P_(CINT) is determined as the product of P_(CO) and CO_(RATIO) by amultiplier 130.

Referring now to FIG. 6, the clutch module 84 will be described indetail. The clutch data includes an active ready control factor and aback stop position. The clutch module 84 includes a force calculatingmodule 132, a slip speed calculating module 134, a friction module 136and a torque calculating module 138. The force calculating module 132determines F_(RES) and a clutch force (F_(CLUTCH)) based on the clutchdata, a nominal kiss point (KP_(NOM)), P_(CINT) and a kiss pointcorrection (KP_(CORR)). More particularly, P_(CINT) is corrected basedon KP_(CORR). KP_(CORR) is continuously updated to account fortolerances and wear in the clutch. F_(CLUTCH) is determined from aseries of look-up tables based on the corrected P_(CINT). F_(CLUTCH) isdetermined from test data averaged from various torque biasing systemsinstrumented to measure force at the clutch based on actuator position.Because there is normally a difference between engaging and releasing(i.e., hysteresis) multiple traces are collected. The direction oftravel determines which table is used and filtering is applied to ensuresmooth transitions.

F_(CLUTCH) is further determined based on a negative clutch force(F_(CLUTCHNEG)), the corrected P_(CINT), KP_(NOM) and the active readycontrol factor. F_(CLUTCHNEG) is a fictitious number that implies thatthe “actual” torque at the clutch is negative when the system is belowthe kisspoint of the clutch. In this manner, the system is maintained atthe active ready position when there is a low torque request. This isachieved by providing a significant control error if the position isbelow the kisspoint. Without F_(CLUTCHNEG), the system would calculatezero torque for any position below the kisspoint causing minimal controlerror for low torque requests regardless of position. Additionally,F_(CLUTCHNEG) is a direct gain on position below kisspoint and is tunedfor optimum response and stability. KP_(NOM) is a constant that isstored in memory and indicates the nominal kiss point (i.e., the pointat which the clutch plates engage) for the particular clutch model.F_(CLUTCH) is calculated as the difference of F_(CLUTCHINT) andF_(CLUTCHNEG).

The slip speed calculating module 134 calculates wheel slip (v_(SLIP))based on the wheel speed signals generated by the sensor group 56. Thefriction calculating module 138 calculates a coefficient of friction(K_(FRICT)) based on F_(CLUTCH), v_(SLIP) and C_(TEMP). Moreparticularly, the friction module 136 determines K_(FRICT) from athree-dimensional look-up table based on F_(CLUTCH), v_(SLIP) andC_(TEMP). The torque calculating module 138 calculates T_(CALC) based onK_(FRCIT) and F_(CLUTCH). T_(CALC) is determined according to thefollowing equation:T _(CALC) =F _(CLUTCH) *N _(PLATES) *R _(EFF) *K _(FRICT)where N_(PLATES) is the number of clutch plates and R_(EFF) is theeffective radius of the clutch plates. N_(PLATES) and R_(EFF) areconstants based on clutch geometry. No hysteresis is assumed.

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

1. A torque biasing system, comprising: a clutch pack; a motor thatmanipulates engagement of said clutch pack based on a control signal;and a control module that generates said control signal based on atorque command and a calculated torque, wherein said calculated torqueis based on a calculated interconnection position of said clutch packand wherein said calculated interconnection position is based on a modelof said torque biasing system.
 2. The torque biasing system of claim 1further comprising a clutch operator mechanism that is driven by saidmotor and that imparts a linear force on said clutch pack.
 3. The torquebiasing system of claim 1 wherein said control signal is based on adifference between said torque command and said calculated torque. 4.The torque biasing system of claim 1 wherein said model of said torquebiasing system includes a motor module, a clutch operator module and aclutch pack module.
 5. The torque biasing system of claim 4 wherein saidmotor module generates a first position signal based on said controlsignal, a motor armature position, a motor temperature, a motor current,motor data and a resistance torque generated by said clutch operatormodule.
 6. The torque biasing system of claim 4 wherein said clutchoperator module generates a second position signal and a resistancetorque based on a first position signal generated by said motor module,clutch operator data and a resistance force generated by said clutchmodule, said second position signal corresponding to said calculatedinterconnection position.
 7. A torque biasing system comprising: aclutch pack; a motor that manipulates engagement of said clutch packbased on a control signal; and a control module that generates saidcontrol signal based on a torque command and a calculated torque,wherein said calculated torque is determined based on a model of saidtorque biasing system and wherein said motor includes a position sensorthat generates an armature position signal, a temperature sensor thatgenerates a temperature signal and a current sensor that generates acurrent signal, wherein said calculated torque is determined based onsaid armature position signal, said temperature signal and said currentsignal.
 8. A torque biasing system comprising: a clutch pack; a motorthat manipulates engagement of said clutch pack based on a controlsignal; and a control module that generates said control signal based ona torque command and a calculated torque, wherein said calculated torqueis determined based on a model of said torque biasing system and whereinsaid clutch pack includes a temperature sensor that generates atemperature signal, wherein said calculated torque is determined basedon said temperature signal.
 9. A torque biasing system comprising: aclutch pack; a motor that manipulates engagement of said clutch packbased on a control signal; and a control module that generates saidcontrol signal based on a torque command and a calculated torque,wherein said calculated torque is determined based on a model of saidtorque biasing system, wherein said model of said torque biasing systemincludes a motor module, a clutch operator module and a clutch packmodule, and wherein said clutch module determines said calculated torqueand a resistance force based on a second position signal generated bysaid clutch operator module, clutch data, a clutch temperature andclutch kiss point data.
 10. A method of controlling a torque biasingsystem, comprising: generating a torque command; calculating aninterconnection position of a clutch of said torque biasing system basedon a model of said torque biasing system; determining a calculatedtorque based on said calculated interconnection position; determining acontrol signal based on said torque command and said calculated torque;and controlling said torque biasing system based on said control signal.11. The method of claim 10 wherein said control signal is based on adifference between said torque command and said calculated torque. 12.The method of claim 10 further comprising generating a first positionsignal in a motor module based on said control signal, a motor armatureposition, a motor temperature, a motor current, motor data and aresistance torque generated by a shift system module.
 13. The method ofclaim 10 further comprising generating a second position signalcorresponding to said calculated interconnection position and aresistance torque in a clutch operator module based on a first positionsignal generated by a motor module, clutch operator data and aresistance force generated by a clutch module.
 14. A method ofcontrolling a torque biasing system, comprising: generating a torquecommand; determining a calculated torque based on a model of said torquebiasing system; determining a control signal based on said torquecommand and said calculated torque; and controlling said torque biasingsystem based on said control signal, wherein said calculated torque isdetermined based on an armature position, a motor temperature and amotor current.
 15. A method of controlling a torque biasing system,comprising: generating a torque command; determining a calculated torquebased on a model of said torque biasing system; determining a controlsignal based on said torque command and said calculated torque; andcontrolling said torque biasing system based on said control signal,wherein said calculated torque is determined based on a clutchtemperature.
 16. A method of controlling a torque biasing system,comprising: generating a torque command; determining a calculated torquebased on a model of said torque biasing system; determining a controlsignal based on said torque command and said calculated torque; andcontrolling said torque biasing system based on said control signal; anddetermining said calculated torque and a resistance force in a clutchmodel based on a second position signal generated by a clutch operatormodule, clutch data, a clutch temperature and kiss-point data.
 17. Amethod of controlling a torque biasing system, comprising: determining atorque command; calculating a model-based torque based on a calculatedinterconnection position of a clutch of said torque biasing system;calculating a torque error based on said torque command and saidmodel-based torque; generating a control signal based on said torqueerror; and operating said torque biasing system based on said controlsignal.
 18. The method of claim 17 further comprising processing aprevious control signal through a torque biasing system model togenerate said model-based torque.
 19. The method of claim 18 whereinsaid torque biasing system model includes a motor model, a clutchoperator model and a clutch model.
 20. The method of claim 19 furthercomprising processing said control signal through said motor model togenerate a clutch operator interconnection value.
 21. The method ofclaim 20 wherein said clutch operator interconnection value is generatedbased on a resistance torque, a motor position signal and motor data.22. The method of claim 21 further comprising calculating saidresistance torque using said clutch operator model.
 23. The method ofclaim 19 further comprising processing an interconnection position valuethrough said clutch operator model to generate said calculatedinterconnection position of said clutch.
 24. The method of claim 23wherein said calculated interconnection position is generated based on aresistance force and clutch operator data.
 25. The method of claim 24further comprising calculating said resistance force using said clutchmodel.
 26. The method of claim 19 further comprising processing a saidcalculated interconnection position through said clutch model togenerate said model-based torque.
 27. A controller for a torque biasingsystem including a clutch and a motor that manipulates engagement ofsaid clutch via a clutch operator, the controller comprising: a motorcontrol module that generates a motor control signal; a motor modulethat generates a calculated clutch operator position signal based onsaid motor control signal; a clutch operator module that generates acalculated clutch interconnection signal based on said calculated clutchoperator position signal; a clutch module that generates a calculatedtorque signal based on said calculated clutch interconnection signal;wherein said motor control signal is based on said calculated torquesignal.
 28. The controller of claim 27 wherein said calculated clutchoperator position signal is generated by said motor module based onmotor data including at least one of: current to torque conversionfactor data, back EMF constant data, brake-on drag data, brake-off dragdata, viscous drag data, coil resistance data, inertia data, and gearratio data.
 29. The controller of claim 27 wherein said calculatedclutch operator position signal is generated by said motor module basedon at least one of: motor position data, motor temperature data, andmotor electrical current data.
 30. The controller of claim 27 whereinsaid calculated clutch interconnection signal is generated by saidclutch operator module based on clutch operator data including at leastone of: spring rate data, efficiency data, drag factor data, viscousdamper drag factor data, position ratio data, and inertia data.
 31. Thecontroller of claim 27 wherein said calculated torque signal isgenerated by said clutch module based on at least one of: negativeclutch force data, nominal kiss point data, kiss point correction data,number of clutch plates data, effective clutch plate radius data, clutchgeometry data, clutch temperature data, and wheel slip data.
 32. Thecontroller of claim 27 wherein said motor control module receives atorque command signal and generates said motor control signal based on adifference between said torque command signal and said calculated torquesignal.